Preparation method of positive electrode material with core-shell structure

Abstract

The invention provides a preparation method of a positive electrode material with a core-shell structure, and relates to the technical field of batteries, the method comprises the following steps: providing a core precursor for generating layered oxide and a coating layer precursor for forming an inorganic coating layer; the coating layer precursor is a micelle-shaped precursor prepared by a sol-gel method; mixing the inner core precursor and the micelle-shaped precursor, drying and dispersing to obtain a composite material precursor; and calcining the composite material precursor, and annealing to obtain the positive electrode material with the core-shell structure. The positive electrode material obtained by the preparation method provided by the invention is relatively high in quality consistency and relatively uniform in coating.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of batteries, in particular to a preparation method of a positive electrode material with a core-shell structure. BACKGROUND

[0002] The positive electrode material of a sodium ion battery accounts for more than 1 / 3 of the cost of the sodium ion battery, and is one of the key components that affect the electrical performance, safety performance, service life and other performances of the sodium ion battery. Therefore, improving the production cost or improving the production efficiency and production quality of the positive electrode material of the sodium ion battery has become one of the keys to reducing the cost of the sodium ion battery and improving the performance of the sodium ion battery.

[0003] In the prior art, the main preparation method for coating the positive electrode material with inorganic matter to form a composite positive electrode material with a core-shell structure is a segmented sintering preparation method, which specifically includes first preparing a fast ion conductor to sinter to form an inner core, and then sintering after coating a shell layer on the surface of the fast ion conductor to obtain a composite positive electrode material with a core-shell structure. SUMMARY

[0004] The purpose of the application is to provide a preparation method of a positive electrode material with a core-shell structure, which has high quality consistency and uniform coating.

[0005] In a first aspect, the application provides a preparation method of a positive electrode material with a core-shell structure, which includes the following steps: providing an inner core precursor for generating a layered oxide and a coating layer precursor for forming an inorganic coating layer; the coating layer precursor is a micellar precursor prepared by a sol-gel method; mixing the inner core precursor and the micellar precursor, drying and dispersing to obtain a composite material precursor; calcining the composite material precursor and annealing to obtain a positive electrode material with a core-shell structure; The calcination includes a first calcination section and a second calcination section, the sintering atmosphere of the first calcination section is a first sintering atmosphere; the first sintering atmosphere includes a first protective gas and oxygen, such as nitrogen and oxygen, and the mass percentage of oxygen in the first sintering atmosphere is 5%-8.5%; the sintering atmosphere of the second calcination section is a second sintering atmosphere; the second sintering atmosphere includes a second protective gas, carbon dioxide and gaseous water, the mass percentage of carbon dioxide in the second sintering atmosphere is 3.2%-5.5%; and the mass percentage of gaseous water in the second sintering atmosphere is 1.0%-2.2 wt%.

[0006] Further, in some embodiments of the present application, the sintering temperature of the first calcination stage is higher than the sintering temperature of the second calcination stage, and the difference between the sintering temperatures of the first and second calcination stages is not higher than 200℃.

[0007] Further, in some embodiments of the present application, the sintering temperature of the first calcination stage is 850-900℃; the sintering temperature of the second calcination stage is 700-750℃; and / or the sintering time of the first calcination stage is 3-6h; and the sintering time of the second calcination stage is 4-8h.

[0008] Further, in some embodiments of the present application, the sum of the sintering time of the first calcination stage and the sintering time of the second calcination stage is not more than 10h.

[0009] Further, in some embodiments of the present application, the size of the micellar particles in the micellar precursor is 10-20nm; and / or the water content in the micellar precursor is 20-50 wt%.

[0010] Further, in some embodiments of the present application, the micellar precursor and the inner core precursor are mixed by ultrasonic mixing; and the solid content in the mixed system formed after mixing is 45-55 wt%.

[0011] Further, in some embodiments of the present application, between the process of mixing the inner core precursor and the micellar precursor and the drying process, there is further included a concentration process; the concentration process includes: concentrating the mixed system to a solid content of 60-65% in the system at 80-85℃.

[0012] Further, in some embodiments of the present application, the annealing temperature is 300-350℃, the annealing time is 2-4h, and the annealing atmosphere is Ar and / or nitrogen. Further, in some embodiments of the present application, the inner core precursor is provided by a method comprising the following steps: providing a first sodium source, a first metal source, a first solvent and a dispersant; mixing the sodium source, the metal source, the solvent and the dispersant, and grinding and spray drying to obtain an inner core precursor; and / or the micellar precursor is provided by a method comprising the following steps: providing a second sodium source, an iron source, a second solvent and a complexing agent, and mixing the second sodium source, the iron source, the solvent and the complexing agent to obtain a coated layer precursor micelle.

[0013] Further, in some embodiments of the present application, the first sodium source is selected from at least one of sodium carbonate, sodium hydroxide and sodium dihydrogen phosphate. the metal source is selected from at least one of an iron source, a nickel source, a manganese source, a copper source, a cobalt source, a titanium source, a magnesium source, an aluminum source, a zinc source, a calcium source; the solvent is selected from at least one of deionized water, ethylene glycol, N-methyl pyrrolidone, ethanol; the dispersant is selected from at least one of polyacrylic acid, ethylene glycol, polyvinyl alcohol; the second sodium source is selected from at least one of sodium carbonate, sodium hydroxide, sodium dihydrogen phosphate; the iron source is selected from at least one of ferric nitrate, iron oxide, ferric phosphate, ferrous sulfate, ferrous oxalate dihydrate, iron; the second solvent is selected from at least one of deionized water, ethylene glycol, N-methyl pyrrolidone, ethanol; the complexing agent is selected from at least one of citric acid, oxalic acid, glycine, polyacrylic acid.

[0014] The application provides a preparation method of a core-shell structure positive electrode material. A precursor particle of an inner core and a gel-like coating layer precursor of a coating layer for coating are formed in advance, and then the core-shell structure positive electrode material is obtained by one-time calcination growth in a segmented and segmented dynamic atmosphere during calcination. The core-shell structure of the material is clear, and the coating uniformity of the coating layer is good. In addition, the inner core material and the coating layer do not need to be sintered respectively, the required sintering time is shorter, the energy consumption is lower, and the production cost is reduced. The crystal growth of the inner core and the coating layer is facilitated, and the charge and discharge capacity and the consistency of the charge and discharge capacity of the positive electrode material are improved. BRIEF DESCRIPTION OF DRAWINGS

[0015] In order to more clearly illustrate the specific embodiments of the application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the description of the specific embodiments or the prior art. Obviously, the drawings described below are some embodiments of the application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0016] Figure 1 SEM image of the positive electrode material obtained in Example 1 of the application; Figure 2 SEM image of the positive electrode material obtained in Comparative Example 1 of the application. DETAILED DESCRIPTION

[0017] The technical solutions of the present application will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of the present application.

[0018] In the description of the present application, it is understood that the meaning of "a plurality of" is two or more than two, unless otherwise explicitly and specifically limited.

[0019] The following disclosure provides many different embodiments or examples for implementing different structures of the present application. For simplicity of the present application's disclosure, the components and settings of specific examples are described below. Of course, they are only examples and the purpose is not to limit the present application. In addition, the present application can repeatedly refer to numbers and / or letters in different examples. Such repetition is for the purpose of simplification and clarity, and in itself does not indicate the relationship between the various embodiments and / or settings discussed. In addition, the present application provides examples of various specific processes and materials, but those of ordinary skill in the art can realize the application of other processes and / or the use of other materials.

[0020] The preparation of the coating layer of the core-shell structure positive electrode material provided in the prior art includes one-time sintering method, two-time sintering method and two-time or more sintering method. At present, the one-time sintering method is mainly used for carbon-coated positive electrode materials and organic-coated positive electrode materials. Among them, the carbon-coated positive electrode material is prepared by mixing the carbon source forming the coating layer and other raw materials forming the inner core layer together to form a precursor, and then sintering the precursor to form a positive electrode material with a core-shell structure. The positive electrode material with an organic coating layer is prepared by sintering to form an inner core and then coating an organic layer. The inorganic coating layer other than the carbon coating layer is usually prepared by two-time sintering or multiple-time sintering, that is, the inner core and the coating material (usually containing a sintering process) are prepared respectively, and then the coating material is coated on the inner core to sinter a positive electrode material with a core-shell structure. In the process of preparing the positive electrode material of NFPP (sodium iron phosphate pyrophosphate) coated O3 phase layered oxide, the applicant found that the coating layer of NaM oxygen@NFPP prepared based on the above process has poor uniformity and poor quality consistency, and the production cost is high. Based on this, the present application provides a preparation method of a positive electrode material with a core-shell structure, which forms a spherical or spherical-like inner core precursor and a gel-like coating layer precursor respectively, coats the coating layer precursor on the surface of the inner core precursor, and sintering to make the inner core and the coating layer grow in situ to obtain a positive electrode material with a core-shell structure. In this process, the sintering atmosphere is controlled to promote the growth of the crystals of the inner core and the coating layer, so that the obtained positive electrode material with a core-shell structure not only has a uniform coating layer, but also has good quality consistency, and the required cost is lower than that of two-time sintering.

[0021] Specifically, the application provides a preparation method of a core-shell structured positive electrode material, comprising the following steps: providing a core precursor for generating a layered oxide and a coating layer precursor for forming an inorganic coating layer; the coating layer precursor is a micellar precursor prepared by a sol-gel method; mixing the core precursor and the micellar precursor, drying and dispersing to obtain a composite precursor; calcining the composite precursor and annealing to obtain the core-shell structured positive electrode material; The calcining comprises a first calcining stage and a second calcining stage, the sintering atmosphere of the first calcining stage is a first sintering atmosphere; the first sintering atmosphere comprises a first protective gas and oxygen, the mass ratio of the oxygen in the first sintering atmosphere is 5%-8.5%; the sintering atmosphere of the second calcining stage is a second sintering atmosphere; the second sintering atmosphere comprises a second protective gas, carbon dioxide and gaseous water, the mass ratio of the carbon dioxide in the second sintering atmosphere is 3.2%-5.5%; the mass ratio of the gaseous water in the second sintering atmosphere is 1.0%-2.2wt%.

[0022] It should be noted that the core precursor in the application refers to a spherical or spherical-like precursor particle to form a core structure, and the core crystal is also grown in situ in the subsequent sintering process; the coating layer precursor is a micellar precursor with certain fluidity, which can uniformly coat the surface of the core precursor during the mixing of the core precursor and the coating layer precursor, and the coating layer is also grown in situ in the subsequent sintering process. That is, in the preparation method provided by the application, the crystals of the core and the coating layer are both grown in situ in the sintering process.

[0023] The product of the positive electrode material prepared by the preparation method provided by the application has good uniformity of coating, which is beneficial to the improvement of its performance, and the quality consistency is good, and the capacity fluctuation between different batches of products is <±1.5%. In addition, the electrical performance of the core-shell structured positive electrode material prepared by the preparation method is better than that of the core-shell structured positive electrode material prepared by secondary sintering. The reason may be that: The preparation method, the inner core and the coating layer are grown in situ in the same sintering process in the application, without damage and uncontrollability of the inner core material and the coating layer material after the preparation of the inner core and the coating layer, and without the dispersion process, so that the quality consistency and electrical properties of the obtained positive electrode material are improved. In addition, the mechanical properties of the core-shell structure positive electrode material provided in the application are also better, especially not prone to cracking. This is because the precursors of the inner core and the coating layer form the coating layer before sintering, and then generate the core-shell structure in situ, so that a certain transition zone is formed on the interface of the core-shell structure during the growth of the inner core and the shell, the bonding force between the coating layer and the inner core is enhanced, and the improvement effect of the coating layer on the volume expansion and other defects of the inner core is more obvious.

[0024] In some embodiments, the first protective gas is selected from at least one of nitrogen, helium, argon, neon and other inert gases; and the second protective gas is selected from at least one of helium, argon, neon and other inert gases. In order to reduce the cost, the first protective gas can be selected as nitrogen.

[0025] In the first calcination stage, the application adopts a specific sintering atmosphere of (91.5%-95%) nitrogen-(5%-8.5%) oxygen, which is beneficial to the oxidation of Ni 2+ to Ni 3+ O, avoids incomplete oxidation or unstable lattice distortion structure. In the second calcination stage, an atmosphere of inert gas with 3.2%-5.5% carbon dioxide and 1.0%-2.2wt% gaseous water is used as the sintering atmosphere, which uses carbon dioxide to control the local pH value≈7, while inhibiting the hydrolysis of iron 2+ , and the gaseous water molecules can promote the solid phase reaction and accelerate the diffusion of iron 2+ , which is beneficial to the formation of a uniform and stable core-shell structure.

[0026] In the calcination process, the specific sintering atmosphere is used, so that hydrogen fluoride required in the preparation process of traditional positive electrode materials is not needed when the core-shell structure positive electrode material is calcined, so that the positive electrode material obtained by the preparation method provided in the application does not need to be treated by hydrogen fluoride, the use of the toxic solution of hydrogen fluoride solution is reduced, which is friendly to the environment, and the toxicity and corrosion of hydrogen fluoride to the equipment are avoided.

[0027] In some embodiments of the present application, the sintering temperature of the first calcination stage is higher than that of the second calcination stage, and the difference between the sintering temperatures of the first and second calcination stages is not higher than 200°C. Specifically, the sintering temperature of the first calcination stage is 850-900°C, and the sintering temperature of the second calcination stage is 700-750°C. The sintering time of the first calcination stage is 3-6h, and the sintering time of the second calcination stage is 4-8h. Preferably, the sintering temperature of the first calcination stage is 840-850°C, the sintering temperature of the second calcination stage is 720-730°C, the sintering time of the first calcination stage is 2-3h, and the sintering time of the second calcination stage is 4-6h.

[0028] Further, in the preparation method provided by the present application, the sum of the sintering time of the first calcination stage and the sintering time of the second calcination stage can be controlled within the range of 6-9h, and the positive electrode material with good quality and good consistency can be obtained within this sintering time range. Compared with other preparation methods for forming a core-shell structure positive electrode material through step-by-step calcination, the preparation method provided by the present application requires a shorter sintering time, which can improve the preparation efficiency and reduce the preparation cost. Specifically, the sum of the sintering time of the first calcination stage and the sintering time of the second calcination stage is not more than 9h, and preferably not more than 8h.

[0029] In some embodiments, the size of the micellar particles in the micellar precursor is 10-20nm, so that they can uniformly coat the surface of the inner core precursor particles to form a precursor coating layer, which is conducive to the in-situ growth and uniform coating of the coating layer in the subsequent calcination process.

[0030] In some embodiments, the water content in the micellar precursor is 20-50 wt%, which provides sufficient fluidity to the micellar precursor, and is conducive to the dispersion of the inner core precursor particles and the uniform coating of the coating layer precursor. In the present application, the viscosity of the micellar precursor can be controlled within the range of 100-500mPa·s, preferably 100-500mPa·s, which is conducive to the stability of the dispersed inner core precursor particles and the mixed system formed by mixing, and avoids agglomeration and sedimentation of the mixed system during the process operation.

[0031] In some embodiments, the micellar precursor and the inner core precursor are mixed by ultrasonic mixing, and the solid content in the mixed system formed after mixing is 45-55wt%, preferably 50-52%. The solid content in the mixed system should not be too high or too low. Too high solid content can easily lead to uneven dispersion of the inner core precursor, long dispersion time, agglomeration of the micellar precursor, and destruction of the micelles during ultrasonic process. Too low solid content can easily lead to agglomeration of the micelles during the concentration process, which in turn affects the thickness uniformity of the coating layer.

[0032] It should be noted that the solid content in the mixed system in the present application is the mass ratio of the sum of the mass of the micellar precursor and the core precursor in the mixed system to the mass in the mixed system.

[0033] In some embodiments, a concentration process is further included between the process of mixing the core precursor and the micellar precursor and the drying process; the concentration process comprises: concentrating the mixed system to a solid content of 60-65% at 80-85°C, so that the water content in the mixed system is removed to below 40%, which is beneficial to the uniform coating of the micelles on the core, avoids the problem of increased impedance caused by the poor uniformity of the micelles due to the direct drying of the mixed system, and further reduces the drying energy consumption and cost.

[0034] In some embodiments, the annealing temperature is 300-350°C, the annealing time is 2-4h, and the annealing atmosphere is argon and / or nitrogen. The core precursor provided by the present application is a core precursor for forming a layered oxide, and the required raw materials can be adjusted according to the structural formula of the layered oxide to be formed. The micellar precursor provided by the present application for forming a coating layer can be adjusted in raw materials according to the structural formula of the coating layer to be formed.

[0035] In some embodiments, the core precursor is provided by a method comprising the following steps: A first sodium source, a first metal source, a first solvent and a dispersing agent are provided; the sodium source, the metal source, the solvent and the dispersing agent are mixed, ground and spray dried to obtain a core precursor.

[0036] The micellar precursor is provided by a method comprising the following steps: A second sodium source, an iron source, a second solvent and a complexing agent are provided; the second sodium source, the iron source, the solvent and the complexing agent are mixed to obtain a coating layer precursor micelle.

[0037] In some embodiments, the first sodium source is selected from at least one of sodium carbonate, sodium hydroxide and sodium dihydrogen phosphate; The metal source is selected from at least one of an iron source, a nickel source, a manganese source, a copper source, a cobalt source, a titanium source, a magnesium source, an aluminum source, a zinc source and a calcium source; The solvent is selected from at least one of deionized water, ethylene glycol, N-methyl pyrrolidone and ethanol; The dispersing agent is selected from at least one of polyacrylic acid, ethylene glycol and polyvinyl alcohol; The second sodium source is selected from at least one of sodium carbonate, sodium hydroxide and sodium dihydrogen phosphate; The iron source is selected from at least one of ferric nitrate, iron oxide, ferric phosphate, ferrous sulfate, ferrous oxalate dihydrate and elemental iron; The second solvent is selected from at least one of deionized water, ethylene glycol, N-methyl pyrrolidone, and ethanol. The complexing agent is selected from at least one of citric acid, oxalic acid, glycine, and polyacrylic acid.

[0038] Exemplarily, the preparation method provided in the present application specifically comprises the following steps: Preparation of the layered oxide precursor The core materials such as sodium source, iron source, nickel source, manganese source, and dopant (if any, if not, no need to add dopant) are provided in a molar ratio of 0.8-1:0.2-0.4:0.2-0.4:0.2-0.4, the core materials are mixed with the first solvent and the dispersion liquid, and the Zeta potential is adjusted to +35 mV or more by the added dispersion liquid, and then grinded, spray dried to obtain the layered oxide precursor particles; (2) Preparation of the sodium iron phosphate pyrophosphate precursor The shell layer materials are provided in a molar ratio of 3.5-4:2.5-3:3.8-4.2 for the sodium source, iron source, and phosphorus source, and the shell layer materials are stirred with the second solvent in a water bath at 40-90°C for 4-24h to obtain a transparent sol-like sodium iron phosphate pyrophosphate precursor; (3) The sodium iron phosphate pyrophosphate precursor and the layered oxide precursor particles are mixed in a mass ratio of 1:10-50, ultrasonically dispersed for 30-180min, and concentrated at 80-85°C to increase the weight of the sodium iron phosphate pyrophosphate precursor by 5-8%, so that the sodium iron phosphate pyrophosphate precursor is uniformly attached to the surface of the layered oxide precursor particles to obtain a composite material; (4) Sintering The composite material is vacuum dried for 4-24h, and then heated to 850-900°C under the first protective gas and 5%-8.5% oxygen, and sintered for 3-6h, and then the sintering atmosphere is adjusted to a mixed atmosphere of the second protective gas, 3.2%-5.5% carbon dioxide, and 1.0%-2.2wt% gaseous water, and then sintered at 700-750°C for 4-8h, and annealed at 200-300°C for 2-8h to obtain the core-shell structure positive electrode material in the present application.

[0039] Preferably, the sodium source in step (1) can be the same as in step (2) to reduce the introduction of more impurities.

[0040] In order for those skilled in the art to better understand the innovations of the present application, the technical solutions of the present application are further described in detail below in combination with examples. The examples of the present application described in detail below are exemplary and are only used to explain the present application, and cannot be understood as a limitation on the present application.

[0041] Example 1 The embodiment provides a preparation method of a core-shell structure positive electrode material, and comprises the following steps: (1) preparing a layered oxide precursor Sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide and copper oxide are provided in a molar ratio of Na:Ni:Fe:Mn:Cu=1:0.4:0.3:0.2:0.1, and the sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide and copper oxide and an ethanol solution are added to a planetary ball mill, an ammonium polyacrylate dispersant is added, the Zeta potential is adjusted to +40 mV or higher, the rotation speed is 400 rpm, and grinding is performed for 24 hours to obtain wet grinding material; the wet grinding material is spray dried under the conditions of an air inlet temperature of 220-250 DEG C, an air outlet temperature of 80-100 DEG C, an atomization pressure of 0.3-0.5 MPa, a nozzle diameter of 0.5 mm, and a peristaltic pump control feeding rate of 10 mL / min to obtain spherical inner core precursor particles with an average particle size of D50: 8 μm.

[0042] (2) preparing a sodium iron pyrophosphate phosphate precursor Ferrous oxalate dihydrate and sodium pyrophosphate are dissolved in a 0.1 mol / L citric acid solution in a 3:1 molar ratio, and stirred in a 80 DEG C water bath for 4 hours to form a blue-green transparent sol, thereby obtaining a nano-micellar sodium iron pyrophosphate phosphate precursor; wherein the average particle size of the micelles is 10-20 nm, the water content is 35%, and the viscosity is 200 mPa·s.

[0043] (3) the inner core precursor particles are added to the transparent sol in a mass ratio of the inner core precursor to the sodium iron pyrophosphate phosphate precursor of 10:1, ultrasonic dispersion is performed for 30 minutes, and 80 DEG C reduced pressure concentration is performed until the sol completely coats the surface of the particles (control the weight gain to be 5-8%); the sodium iron pyrophosphate phosphate precursor is uniformly attached to the surface of the inner core precursor particles, thereby obtaining a composite material; (4) sintering The composite material is vacuum dried at 100 DEG C for 12 hours, nitrogen and oxygen are introduced into a calcination device, the flow rate of the nitrogen is 9.5 L / min, the flow rate of the oxygen is 0.5 L / min, a calcination atmosphere formed by the nitrogen and the oxygen is 95% nitrogen+5% oxygen, and the gas pressure is maintained in the range of 0.5-0.6 MPa; the temperature is increased to 850 DEG C, and sintering is performed for 4 hours; argon is introduced into the calcination device to replace the sintering atmosphere in the device, and water-containing carbon dioxide gas is introduced, so that the atmosphere in the device is 93% argon+5% carbon dioxide+2% water, the gas pressure is 0.3-0.5 MPa, the temperature is increased to 750 DEG C, sintering is performed for 6 hours, the temperature is decreased to 300 DEG C, and annealing is performed for 2 hours, thereby obtaining the positive electrode material NaNi 0.4 Fe 0.3 Mn 0.3 Cu 0.1O2@1 / 11Na4Fe3(PO4)2(P2O7), which has a morphology as shown in Figure 1

[0044] Five batches of positive electrode materials were prepared according to the above preparation method, and samples S11-S15 were taken for standby.

[0045] Example 2 The present embodiment provides a preparation method of a core-shell structure positive electrode material, comprising the following steps: (1) Preparation of layered oxide precursor Sodium carbonate, nickel hydroxide, ferrous oxalate dihydrate, manganese sesquioxide, and cerium sesquioxide were provided in a molar ratio of Na:Ni:Fe:Mn:Ce = 1:0.3:0.3:0.3:0.1. The sodium carbonate, nickel hydroxide, ferrous oxalate dihydrate, manganese sesquioxide, and cerium sesquioxide were added to a planetary ball mill, and a PVP dispersant was added to adjust the Zeta potential to +40 mV or higher. The rotation speed was 400 rpm, and the grinding time was 24 hours to obtain a wet grinding material. The wet grinding material was spray dried under the following conditions: inlet temperature: 220-250°C, outlet temperature: 80-100°C, atomization pressure: 0.3-0.5 MPa, nozzle diameter: 0.5 mm, and peristaltic pump control feed rate: 10 mL / min. The resulting spherical inner core precursor particles had an average particle size of D50: 10 μm.

[0046] (2) Preparation of sodium iron phosphate pyrophosphate precursor Ferrous oxalate dihydrate and sodium pyrophosphate were dissolved in a 0.1 mol / L citric acid solution at a molar ratio of 3:1 and stirred in a 80°C water bath for 4 hours to form a blue-green transparent sol, obtaining a nano-micellar sodium iron phosphate pyrophosphate precursor. The average particle size of the micelles was 10-20 nm, the water content was 35%, and the viscosity was 200 mPa·s.

[0047] (3) The inner core precursor particles were added to the transparent sol at a mass ratio of inner core precursor: sodium iron phosphate pyrophosphate precursor of 10:1, and ultrasonic dispersion was performed for 30 minutes. The sol was concentrated at 80°C under reduced pressure until the sol completely coated the surface of the particles (weight gain of 5-8%); the sodium iron phosphate pyrophosphate precursor was uniformly attached to the surface of the inner core precursor particles to obtain a composite material. (4) Sintering ​The composite material is vacuum dried at 100℃ for 12 hours, nitrogen and oxygen are introduced into the calcination device, the flow rate of nitrogen is 9.5L / min, the flow rate of oxygen is 0.5L / min, the calcination atmosphere formed by nitrogen and oxygen is 95% nitrogen + 5% oxygen, the gas pressure is maintained in the range of 0.5~0.6Mpa; the temperature is raised to 850℃, and sintering is performed for 4 hours; then argon is introduced into the calcination device to replace the sintering atmosphere in the device, and water-containing carbon dioxide gas is introduced to make the atmosphere in the device 80% argon + 18% carbon dioxide + 2% water, the gas pressure is 0.3~0.5MPa, the temperature is raised to 750℃ and sintering is performed for 6h, and the temperature is lowered to 300℃ and annealing is performed for 2 hours, to obtain the positive electrode material in the present application: NaNi 0.3 Fe 0.3 Mn 0.3 Ce 0.1 O2@1 / 11Na4Fe3(PO4)2(P2O7).

[0048] 5 batches of positive electrode materials are prepared according to the preparation method described above, and samples S21~S25 are taken respectively for standby.

[0049] Example 3 The present embodiment provides a preparation method of a positive electrode material with core-shell structure, comprising the following steps: (1) preparing a layered oxide precursor Sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide and copper oxide are provided in a molar ratio of Na:Ni:Fe:Mn:Cu = 1:0.4:0.3:0.2:0.1, and the sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide and copper oxide and an ethanol solution are added to a planetary ball mill, an ammonium polyacrylate dispersant is added, the Zeta potential is adjusted to +40mV or more, the rotation speed is 400rpm, and grinding is performed for 24 hours to obtain a wet grinding material; the wet grinding material is spray dried under the following conditions: inlet temperature: 220-250℃, outlet temperature: 80-100℃, atomization pressure: 0.3-0.5MPa, nozzle diameter 0.5mm, and peristaltic pump control feeding rate: 10mL / min, to obtain spherical inner core precursor particles with an average particle size of D50: 8μm.

[0050] (2) preparing a sodium iron pyrophosphate phosphate precursor Iron nitrate and sodium pyrophosphate are dissolved in a 0.1mol / L glycine solution in a molar ratio of 3:1, stirred in a 60℃ water bath for 3 hours, and a transparent sol is formed to obtain a nano-micellar sodium iron pyrophosphate phosphate precursor; wherein the average particle size of the micelles is 20~30nm, the water content is 30%, and the viscosity is 100mPa·s.

[0051] (3) The core precursor particles were added to a transparent sol at a mass ratio of 10:1, ultrasonically dispersed for 30 minutes, and concentrated under reduced pressure at 80°C until the sol completely covered the particle surface (weight gain of 5-8%); so that the sodium iron pyrophosphate precursor was uniformly attached to the surface of the core precursor particles to obtain the composite material. (4) Sintering The composite material was vacuum dried at 100°C for 12 hours. Nitrogen and oxygen were then introduced into the calcination equipment at a flow rate of 9.2 L / min and 0.8 L / min, respectively, to create a calcination atmosphere of 92% nitrogen and 8% oxygen, with the pressure maintained between 0.5 and 0.6 MPa. The temperature was then raised to 850°C and sintered for 4 hours. Argon was then introduced into the calcination equipment to replace the sintering atmosphere, followed by the introduction of water-containing carbon dioxide gas to create an atmosphere of 93% argon, 5% carbon dioxide, and 2% water, with a pressure between 0.3 and 0.5 MPa. The temperature was then raised to 750°C and sintered for 6 hours, followed by annealing at 300°C for 2 hours to obtain the cathode material of this application. NaNi 0.4 Fe 0.3 Mn 0.3 Cu 0.1 O2@1 / 11Na4Fe3(PO4)2(P2O7).

[0052] Five batches of cathode materials were prepared according to the above preparation method, and samples S31~S35 were taken from each batch for later use.

[0053] Comparative Example 1 Compared to Example 1, the sintering atmosphere in step (4) of this comparative example is 100% nitrogen for the first calcination and 100% argon for the second calcination. The remaining steps are the same as in Example 1, resulting in a core-shell structured cathode material D1, the morphology of which is as follows. Figure 2 As shown.

[0054] Comparative Example 2 Compared with Example 1, the sintering atmosphere in step (4) of this comparative example is 50% nitrogen-50% oxygen for the first calcination and 50% argon-25% carbon dioxide-25% water for the second calcination. The remaining steps are the same as in Example 1, and a core-shell structured cathode material D2 is obtained.

[0055] Comparative Example 3 This embodiment provides a method for preparing a core-shell structured cathode material, including the following steps: (1) Preparation of layered oxide precursors Sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide and copper oxide are provided in a molar ratio of Na:Ni:Fe:Mn:Cu = 1:0.4:0.3:0.2:0.1, and the sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese sesquioxide and copper oxide and an ethanol solution are added to a planetary ball mill, an ammonium polyacrylate dispersant is added, the Zeta potential is adjusted to +40 mV or more, the rotation speed is 400 rpm, and grinding is performed for 24 hours to obtain a wet mill base; the wet mill base is spray dried under the following conditions: an air inlet temperature of 220-250°C, an air outlet temperature of 80-100°C, an atomization pressure of 0.3-0.5 MPa, a nozzle diameter of 0.5 mm, and a peristaltic pump control feed rate of 10 mL / min to obtain spherical core precursor particles with an average particle size of D50: 10 μm.

[0056] (2) Calcination of the core precursor particles The core precursor particles are calcined at 850°C for 6 hours in an oxygen atmosphere to obtain the layered oxide particles.

[0057] (3) Preparation of sodium iron phosphate pyrophosphate precursor Ferrous oxalate dihydrate and sodium pyrophosphate are dissolved in a 0.1 mol / L citric acid solution at a molar ratio of 3:1, and stirred in a water bath at 80°C for 4 hours to form a blue-green transparent sol, thereby obtaining a nano-micellar sodium iron phosphate pyrophosphate precursor; the average particle size of the micelles is 10-20 nm, the water content is 40%, and the viscosity is 120 mPa·s.

[0058] (4) The layered oxide particles are added to the transparent sol at a mass ratio of 10:1, ultrasonic dispersion is performed for 30 minutes, and the transparent sol is concentrated at 50°C under reduced pressure until the sol completely coats the surface of the particles (weight gain of 5-8%); the sodium iron phosphate pyrophosphate precursor is uniformly attached to the surface of the layered oxide particles to obtain a composite material. (5) Sintering The composite material is vacuum dried at 100°C for 12 hours, nitrogen and oxygen are introduced into the calcination equipment, the flow rate of the nitrogen is 9.5 L / min, the flow rate of the oxygen is 0.5 L / min, the calcination atmosphere formed thereby is 95% nitrogen + 5% oxygen, and the gas pressure is maintained within the range of 0.5-0.6 MPa; the temperature is raised to 850°C, and sintering is performed for 4 hours; argon is introduced into the calcination equipment to replace the sintering atmosphere therein, and water-containing carbon dioxide gas is introduced to make the atmosphere in the equipment 80% argon + 18% carbon dioxide + 2% water, and the gas pressure is 0.3-0.5 MPa; the temperature is raised to 750°C, and sintering is performed for 6 hours; the temperature is lowered to 300°C, and annealing is performed for 2 hours to obtain the positive electrode material in the present application.

[0059] Five batches of the positive electrode material are prepared according to the above preparation method, and samples D31-D35 are taken for standby use.

[0060] Comparative Example 4 This embodiment provides a method for preparing a core-shell structured cathode material, including the following steps: (1) Preparation of layered oxide precursors Sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese trioxide, and copper oxide were provided in a molar ratio of Na:Ni:Fe:Mn:Cu = 1:0.4:0.3:0.2:0.1. These components, along with an ethanol solution, were added to a planetary ball mill. Ammonium polyacrylate dispersant was added, and the Zeta potential was adjusted to above +40mV. The mill was then ground at 400 rpm for 24 hours to obtain wet abrasive. The wet abrasive was then spray-dried under the following conditions: inlet air temperature: 220-250℃; outlet air temperature: 80-100℃; atomization pressure: 0.3-0.5MPa; nozzle diameter: 0.5mm; and peristaltic pump-controlled feed rate: 10mL / min. This yielded spherical core precursor particles with an average particle size of D50: 10μm.

[0061] (2) Calcination of kernel precursor particles The above-mentioned core precursor particles were calcined at 850°C for 6 hours in an oxygen atmosphere to obtain layered oxide particles.

[0062] (3) NFPP particles with an average particle size D50 of 6 μm purchased from Hunan Meite Company were wet-mixed in a batch mixer at a mass ratio of 10:1 with ethanol as the solvent to obtain a mixture O3@NFPP. (4) Sintering The mixture was vacuum dried at 100°C for 12 hours. Nitrogen and oxygen were then introduced into the calcination equipment. The nitrogen flow rate was 9.5 L / min and the oxygen flow rate was 0.5 L / min, creating a calcination atmosphere of 95% nitrogen and 5% oxygen, with the pressure maintained between 0.5 and 0.6 MPa. The temperature was then raised to 850°C and sintered for 4 hours. Argon was then introduced into the calcination equipment to replace the sintering atmosphere. Water-containing carbon dioxide gas was then introduced to create an atmosphere of 93% argon, 5% carbon dioxide, and 2% water, with a pressure between 0.3 and 0.5 MPa. The temperature was raised to 750°C and sintered for 6 hours. The temperature was then lowered to 300°C and annealed for 2 hours to obtain the cathode material of this application.

[0063] Five batches of cathode materials were prepared according to the above preparation method, and samples D41~D45 were taken from each batch for later use.

[0064] The applicant prepared sodium-ion batteries using the core-shell structured positive electrode materials obtained in Examples 1 to 3 and the positive electrode materials of Comparative Examples 1 to 4 as positive electrode active materials, and the preparation was as follows: (1) Preparation of the positive electrode: the positive electrode material, SP, PVDF and CNT were mixed, stirred and kneaded in a mass ratio of 95:1:3:1, and N-methyl pyrrolidone solvent was added. The slurry was adjusted to a positive electrode slurry with a solid content of 60% and a viscosity of 6000 mPa·s. The transfer coating method was used to transfer and coat the positive electrode slurry onto a 15 μm thick carbon-coated aluminum foil. The coated electrode sheet was rolled to a compaction density of 3 mg / cm 3 . The rolled electrode sheet was die-cut into an electrode sheet with a length of 48 mm and a width of 38 mm for standby use.

[0065] (2) Preparation of the negative electrode sheet: the hard carbon material, SP, CMC and SBR were mixed, stirred and kneaded in a mass ratio of 92:3:2:3, and deionized water was added. The slurry was adjusted to a negative electrode slurry with a solid content of 45% and a viscosity of 5000 mPa·s. The transfer coating method was used to transfer and coat the negative electrode slurry onto a 15 μm thick carbon-coated aluminum foil. The coated electrode sheet was rolled to a compaction density of 1 mg / cm 2 . The rolled electrode sheet was die-cut into an electrode sheet with a length of 50 mm and a width of 40 mm for standby use.

[0066] (3) Preparation of the sodium-ion battery: the striped positive electrode sheet and the negative electrode sheet were laminated on a laminator, the separator was a PP / PE / PP three-layer material, a soft package battery cell was formed, and electrolyte (composition: carbonate solvent and 1M sodium hexafluorophosphate) was injected. The battery was packaged, dried, formed and tested for capacity to obtain a sodium-ion battery.

[0067] The above-prepared sodium-ion battery was subjected to morphology characterization test, cycle performance test, charge-discharge capacity test and rate performance test. The specific test methods were as follows: (1) Cycle performance test The cycle performance of the battery was tested by the test method recorded in GB / T 31485-2015.

[0068] (2) Volume expansion rate test The volume change of the battery before and after storage at 55°C for 28 days was tested by the drainage method.

[0069] (3) Charge-discharge capacity test The charge-discharge capacity of the battery was tested by the test method recorded in GB / T 31467.2-2015.

[0070] The test results are shown in Table 1.

[0071] Table 1

[0072] As can be seen from Table 1, the preparation method of the positive electrode material provided by the application adopts segmented sintering in the sintering process, and a specific sintering atmosphere is used in each sintering segment, so that the morphology of the obtained positive electrode material is controlled, the directional growth of the crystals of the core and the shell layer is controlled, and the cycle performance, surface impedance, volume expansion rate and other performances of the positive electrode material are optimized; meanwhile, the uniformity of the coating of the particles of the positive electrode material can be improved, and the quality consistency of the product is further improved.

[0073] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the application, but not to limit them; although the application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the application.

Claims

1. A method for preparing a core-shell structured cathode material, characterized in that, Includes the following steps: A core precursor for generating layered oxides and a coating layer precursor for forming an inorganic coating layer are provided; the coating layer precursor is a micellar precursor prepared by the sol-gel method. The core precursor and the micelle precursor are mixed, dried, and dispersed to obtain the composite material precursor. The composite material precursor is calcined and annealed to obtain a core-shell structured cathode material. The calcination process includes a first calcination section and a second calcination section. The sintering atmosphere of the first calcination section is a first sintering atmosphere, which includes a first protective gas and oxygen, with the oxygen accounting for 5%-8.5% of the total mass. The sintering atmosphere of the second calcination section is a second sintering atmosphere, which includes a second protective gas, carbon dioxide, and gaseous water, with the carbon dioxide accounting for 3.2%-5.5% of the total mass and the gaseous water accounting for 1.0%-2.2 wt%.

2. The method for preparing the core-shell structured cathode material according to claim 1, characterized in that, The sintering temperature of the first calcination section is higher than that of the second calcination section, and the difference between the sintering temperatures of the first calcination section and the second calcination section is not higher than 200℃.

3. The method for preparing the core-shell structured cathode material according to claim 2, characterized in that, The sintering temperature of the first calcination section is 850-900℃; the sintering temperature of the second calcination section is 700-750℃; and / or The sintering time of the first calcination section is 3-6 hours; the sintering time of the second calcination section is 4-8 hours.

4. The method for preparing the core-shell structured cathode material according to claim 3, characterized in that, The sum of the sintering time of the first calcination section and the sintering time of the second calcination section shall not exceed 10 hours.

5. The method for preparing the core-shell structured cathode material according to any one of claims 1 to 4, characterized in that, The size of the micelle particles in the micelle precursor is 10-20 nm; and / or The water content in the micelle precursor is 20-50 wt%.

6. The method for preparing the core-shell structured cathode material according to claim 5, characterized in that, The micelle precursor and the core precursor are mixed by ultrasonication; the solid content in the resulting mixture is 45-55 wt%.

7. The method for preparing the core-shell structured cathode material according to claim 6, characterized in that, Between the process of mixing the core precursor and the micelle precursor and the drying process, there is also a concentration process; The concentration process includes: concentrating the mixture at 80-85°C until the solid content in the system is 60-65%.

8. The method for preparing the core-shell structured cathode material according to claim 1, characterized in that, The annealing temperature is 300~350℃, the annealing time is 2~4h, and the annealing atmosphere is argon and / or nitrogen.

9. A method for preparing a core-shell structured cathode material according to any one of claims 1 to 4, 6, and 7, characterized in that, The kernel precursor is provided by a method comprising the following steps: Provide a first sodium source, a first metal source, a first solvent, and a dispersant; mix the sodium source, metal source, solvent, and dispersant, grind, and spray dry to obtain a core precursor; and / or The micelle precursor is provided by a method comprising the following steps: A second sodium source, an iron source, a second solvent, and a complexing agent are provided. The second sodium source, iron source, solvent, and complexing agent are mixed to obtain a coating layer precursor micelle.

10. The method for preparing the core-shell structured cathode material according to claim 9, characterized in that, The first sodium source is selected from at least one of sodium carbonate, sodium hydroxide, and sodium dihydrogen phosphate; The metal source is selected from at least one of the following: iron source, nickel source, manganese source, copper source, cobalt source, titanium source, magnesium source, aluminum source, zinc source, and calcium source. The solvent is selected from at least one of deionized water, ethylene glycol, N-methylpyrrolidone, and ethanol; The dispersant is selected from at least one of polyacrylic acid, ethylene glycol, and polyvinyl alcohol; The second sodium source is selected from at least one of sodium carbonate, sodium hydroxide, and sodium dihydrogen phosphate; The iron source is selected from at least one of ferric nitrate, ferric oxide, ferric phosphate, ferrous sulfate, ferrous oxalate dihydrate, and elemental iron. The second solvent is selected from at least one of deionized water, ethylene glycol, N-methylpyrrolidone, and ethanol; The complexing agent is selected from at least one of citric acid, oxalic acid, glycine, and polyacrylic acid.

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